It is an important purpose of the invention to provide a small transducer capable of relating a pressure force to a frequency, which may be regulated or generated by light, pneumatic, acoustic, or electrical energy or the equivalents. For the purposes of simplicity and ease of reference, the process will be referred to herein as pressure-to-frequency conversion, but the generic conception for which patent protection is sought includes all physical phenomena which are capable of conversion to a pressure communicable to a resonant hollow beam structure. In general, a measurand is the physical phenomena, i.e., physical quantity, force, property, or condition, which is being measured. Common measurands in the process control industry include flow rate, pressure and differential pressure, level, temperature, and analytical measurements such as pH, oxidation reduction potential (ORP), conductivity, and the like. It is useful to recognize that various physical phenomena may be converted to pressure or pressure variations, including thermal measurands, radiation measurands, force measurands, rate measurands, quantity measurands, specific physical property measurands and electrical measurands.
Among the pressure-to-frequency schemes presently known are the Patents to Ferran et al, U.S. No. 3,187,579; to Cooiman, U.S. No. 3,327,533, and U.S. No. 3,257,850; to Hobbs, U.S. No. 2,466,809; and U.S. No. 3,831,137 to Cuomo. Other vibratable element sensors include vibrating tensioned wire systems as are illustrated by U.S. Pat. Nos. 4,164,651 and 4,149,422.
Common to all of the foregoing are limitations of space due to the large size of the sensing elements, and complexity of the assembled sensor. Also, the prior art is often lacking in flexibility as to the forms of energy powering the oscillatory mechanism and the form of energy by which the resonant frequency is sensed. The present invention comprehends a microstructural resonant hollow cavity beam sensor, and the microfabrication or micromachining process by which it is manufactured. These fabrication techniques permit fabricating large quantities of the sensor at low labor cost and with extremely consistent operating characteristics.
Ordinary and conventional miniaturization techniques enable, with the exercise of considerable care, patience, and skill, the manufacture of hollow resonant elements as small as about ten times greater than this invention (e.g., about one-half inch long). Different materials are used (steel sheet metal); the micro-size resonant element envisioned by the present invention cannot readily be made by such conventional miniaturization techniques.
Two published articles which are especially useful in describing microfabrication and micromachining techniques are K. E. Peterson, "Silicon as a Mechanical Material", Proceedings of the IEEE, Vol. 70, No. 5, May 1982, pp. 420-457 and J. B. Angell, S. C. Terry, and P. W. Barth, "Silicon Micromechanical Devices", Scientific American, Vol. 248, No. 4, April 1983, pp. 44-55.